The identification of analytes eluting from a liquid chromatography column is expedited with the combined use of a particle beam (PB) interface and a mass spectrometer [1,2]. The technique is unique in that although analytes enter the PB interface while in solution, the data usually obtained are electron impact (EI) mass spectra. The EI spectra acquired are library searchable, which further aids in the identification of the molecule. The compounds may also be chemically ionized (CI) [2] for molecular weight determination or for improved signal response in mass spectrometry/mass spectrometry experiments. In addition to these advantages, PB offers an inlet for compounds that are thermally unstable and/or are not volatile enough for efficient GC/MS analysis without chemical modification. The ability to obtain an EI mass spectrum combined with these other benefits has lead many mass spectrometer manufacturers to develop particle beam interfaces for use on quadrupole instruments [3].
Prior to the PB interface, only membrane [4,5,6] and moving belt interfaces [7,8,9,10,11] have allowed for the generation of both EI and CI spectra from an analyte initially in a liquid solution. Dimeethyl-vinyl silicon polymer membrane interfaces have shown high sensitivity, but primarily for volatile nonpolar molecules and thus, one of their main uses has been in the analysis of volatile organics in water and in reaction monitoring [12]. The moving belt apparatus has been successful when looking at analytes in high concentrations of water [13] and has been used to detect molecules in the picogram range [9], but it can have memory effects and is considered cumbersome by many. Other techniques such as thermospray [14], electrospray [15], and flow fast atom bombardment (FAB) [16,17] are used as on-line continuous liquid chromatography interfaces, but they produce "CI-like" mass spectra and also give little structural information. Recently particle beam interfaces have been used to introduce analyte into the matrix during fast atom bombardment ionization (FAB) [18,19]. The sensitivity was low, as expected due to analyte loss in the interface, but the experiment demonstrates a novel method of sample addition.
Although improvements have been made to the particle beam interface [20], the concept of using stages of momentum separation to enrich the beam with particles and to remove solvent molecules has remained unchanged. In PB systems, a liquid solution is nebulized into micrometer-sized droplets which are partially desolvated and accelerated through a nozzle. Once through the nozzle, high velocity particles are momentum separated from solvent molecules by a skimmer and are then transferred to the next stage of momentum separation. The enrichment region has traditionally consisted of two stages of momentum separation followed by a transfer line [1,2,3]. Once separated from the solvent, the particles are transmitted to the ion source of a differentially pumped mass spectrometer for vaporization and ionization. The particle separation does have its limitations, such as changes in the analyte response, due to a coelution carrier process [21] and a non-linear calibration curve which reduces the simplicity of quantitative analysis [2], but the technique is unique in its ability to produce EI spectra.
In liquid chromatography/mass spectrometry (LC/MS), it is crucial to reduce the partial pressure of the solvent or unwanted molecules that reach the mass spectrometer. This becomes even more important in non-differentially pumped quadrupole and ion trap mass spectrometers.
This invention describes a novel three-stage particle beam interface with a gas addition port. The particle separator can be used to couple an LC to an ion trap mass spectrometer. The interface uses an additional stage of momentum separation before transmitting the particles to the ion trap and incorporates the addition of make-up gas to improve jet separation and maintain optimum ion trap helium pressure. Particles pass directly into the trapping chamber, where they are vaporized and ionized to produce EI mass spectra.